U.S. patent number 10,658,659 [Application Number 16/540,755] was granted by the patent office on 2020-05-19 for electroactive materials for metal-ion batteries.
This patent grant is currently assigned to Nexeon Limited. The grantee listed for this patent is Nexeon Limited. Invention is credited to Christopher Michael Friend, Charles Mason, Richard Taylor.
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United States Patent |
10,658,659 |
Mason , et al. |
May 19, 2020 |
Electroactive materials for metal-ion batteries
Abstract
The invention relates to a particulate material comprising a
plurality of composite particles, wherein the composite particles
comprise: (a) a porous carbon framework comprising micropores and
mesopores having a total pore volume of at least 0.6 cm.sup.3/g and
no more than 2 cm.sup.3/g, where the volume fraction of micropores
is in the range from 0.5 to 0.9 and the volume fraction of pores
having a pore diameter no more than 10 nm is at least 0.75, and the
porous carbon framework has a D.sub.50 particle size of less than
20 .mu.m; (b) silicon located within the micropores and/or
mesopores of the porous carbon framework in a defined amount
relative to the volume of the micropores and/or mesopores.
Inventors: |
Mason; Charles (Abingdon,
GB), Taylor; Richard (Abingdon, GB),
Friend; Christopher Michael (Abingdon, GB) |
Applicant: |
Name |
City |
State |
Country |
Type |
Nexeon Limited |
Abingdon, Oxfordshire |
N/A |
GB |
|
|
Assignee: |
Nexeon Limited (Abingdon,
GB)
|
Family
ID: |
65147067 |
Appl.
No.: |
16/540,755 |
Filed: |
August 14, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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16274187 |
Feb 12, 2019 |
10424786 |
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Foreign Application Priority Data
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Dec 19, 2018 [GB] |
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1820695.3 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M
4/134 (20130101); H01M 4/362 (20130101); H01M
4/364 (20130101); H01M 4/366 (20130101); H01M
4/133 (20130101); H01M 4/386 (20130101); H01M
4/0421 (20130101); H01M 4/625 (20130101); H01M
10/0525 (20130101); H01M 2004/021 (20130101); H01M
2004/025 (20130101); H01M 2004/027 (20130101) |
Current International
Class: |
H01M
4/36 (20060101); H01M 10/0525 (20100101); H01M
4/38 (20060101); H01M 4/62 (20060101); H01M
4/02 (20060101) |
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|
Primary Examiner: Godenschwager; Peter F
Attorney, Agent or Firm: McDonnell Boehnen Hulbert &
Berghoff LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. application Ser. No.
16/274,187, filed Feb. 12, 2019, which claims the benefit of
priority of United Kingdom Patent Application no. GB1820695.3,
filed Dec. 19, 2018, which are hereby incorporated herein by
reference in their entirety.
Claims
The invention claimed is:
1. A particulate material comprising a plurality of composite
particles, wherein the composite particles comprise: (a) a porous
carbon framework comprising micropores and mesopores, wherein (i)
the micropores and mesopores have a total pore volume as measured
by gas adsorption of P.sub.1 cm.sup.3/g, wherein P.sub.1 has a
value of at least 0.6 and no more than 1.6, (ii) the volume
fraction of micropores (.phi..sub.a) is in the range from 0.5 to
0.9, based on the total volume of micropores and mesopores; (iii)
the volume fraction of pores having a pore diameter no more than 10
nm (.phi..sub.10) is at least 0.75, based on the total volume of
micropores and mesopores, and (iv) the porous carbon framework has
a D.sub.50 particle size of less than 20 .mu.m; (b) a plurality of
nanoscale elemental silicon domains located within the micropores
and/or mesopores of the porous carbon framework; wherein the weight
ratio of silicon to the porous carbon framework in the composite
particles is in the range from [1.times.P.sub.1 to
1.9.times.P.sub.1]:1.
2. A particulate material according to claim 1, wherein P.sub.1 has
a value of at least 0.65.
3. A particulate material according to claim 1, wherein P.sub.1 has
a value of no more than 1.4.
4. A particulate material according to claim 1, wherein P.sub.1 has
a value of at least 0.65 and no more than 1.4.
5. A particulate material according to claim 1, wherein the volume
fraction of micropores (.phi..sub.a) is in the range from 0.5 to
0.8.
6. A particulate material according to claim 1, wherein the volume
fraction of micropores (.phi..sub.a) is in the range from 0.5 to
0.75.
7. A particulate material according to claim 1, wherein the weight
ratio of silicon to the porous carbon framework in the composite
particles is at least the value given by
[.phi..sub.b+0.75].times.P.sub.1, wherein .phi..sub.b represents
the volume fraction of mesopores, based on the total volume of
micropores and mesopores.
8. A particulate material according to claim 1, wherein the weight
ratio of silicon to the porous carbon framework in the composite
particles is at least the value given by
[.phi..sub.b+1.1].times.P.sub.1.
9. A particulate material according to claim 1, wherein the weight
ratio of silicon to the porous carbon framework in the composite
particles is no more than the value given by
[.phi..sub.b+1.6].times.P.sub.1.
10. A particulate material according to claim 1, wherein the weight
ratio of silicon to the porous carbon framework in the composite
particles is at least the value given by
[.phi..sub.b+1].times.P.sub.1 and is no more than the value given
by [.phi..sub.b+1.5].times.P.sub.1.
11. A particulate material according to claim 1, wherein the weight
ratio of silicon to the porous carbon framework in the composite
particles is at least 1.2.times.P.sub.1.
12. A particulate material according to claim 1, wherein the weight
ratio of silicon to the porous carbon framework in the composite
particles is at least 1.4.times.P.sub.1.
13. A particulate material according to claim 1, wherein the volume
fraction of pores having a pore diameter of no more than 10 nm
(.phi..sub.10) is at least 0.9, based on the total volume of
micropores and mesopores.
14. A particulate material according to claim 13, wherein the
volume fraction of pores having a pore diameter of no more than 5
nm (.phi..sub.5) is at least 0.8, based on the total volume of
micropores and mesopores.
15. A particulate material according to claim 1, wherein the volume
fraction of pores having a pore diameter of no more than 5 nm
(.phi..sub.5) is at least 0.7, based on the total volume of
micropores and mesopores.
16. A particulate material according to claim 1, wherein the porous
carbon framework comprises macropores having a diameter in the
range from greater than 50 nm to 100 nm having a total volume
P.sub.2 cm.sup.3/g as measured by mercury porosimetry, wherein
P.sub.2 is no more than 0.2.times.P.sub.1.
17. A particulate material according to claim 1, wherein the
composite particles have a D.sub.50 particle diameter of at least
0.5 .mu.m and no more than 18 .mu.m.
18. A particulate material according to claim 17, wherein the
composite particles have a D.sub.10 particle diameter of at least
0.2 .mu.m and a D.sub.90 particle diameter of no more than 40
.mu.m.
19. A particulate material according to claim 1, wherein the
composite particles have a BET surface area of no more than 30
m.sup.2/g.
20. A particulate material according to claim 1, wherein P.sub.1 is
in the range from 0.7-1.4; .phi..sub.a is in the range from 0.5 to
0.8; .phi..sub.10 is at least 0.8; and the D.sub.50 particle size
of the porous carbon framework is in the range from 1 to 18
.mu.m.
21. A particulate material according to claim 20, wherein the
weight ratio of silicon to the porous carbon framework is at least
the value given by [.phi..sub.b+0.8].times.P.sub.1 and no more than
the value given by [.phi..sub.b+1.6].times.P.sub.1.
22. A particulate material according to claim 20, wherein the
weight ratio of silicon to the porous carbon framework is in the
range from [1.2.times.P.sub.1 to 1.8.times.P.sub.1]:1.
23. A particulate material according to claim 1, wherein P.sub.1 is
in the range from 0.8-1.2; .phi..sub.a is in the range from 0.6 to
0.8; .phi..sub.10 is at least 0.8; and the D.sub.50 particle size
of the porous carbon framework is in the range from 1 to 18
.mu.m.
24. A particulate material according to claim 23, wherein the
weight ratio of silicon to the porous carbon framework is at least
the value given by [.phi..sub.b+0.8].times.P.sub.1 and no more than
the value given by [.phi..sub.b+1.6].times.P.sub.1.
25. A particulate material according to claim 23, wherein the
weight ratio of silicon to the porous carbon framework is in the
range from [1.2.times.P.sub.1 to 1.8.times.P.sub.1]:1.
26. A particulate material according to claim 1, wherein having a
total oxygen content of no more than 15 wt %.
27. A particulate material according to claim 1, wherein having a
total oxygen content of no more than 10 wt %.
28. A particulate material according to claim 1, wherein having a
total oxygen content of no more than 5 wt %.
29. A particulate material according to claim 1, wherein at least
90 wt % of the silicon mass in the composite particles is located
within the internal pore volume of the porous carbon framework.
30. A composition comprising a particulate material as defined in
claim 1, and at least one other component selected from: (i) a
binder; (ii) a conductive additive; and (iii) an additional
particulate electroactive material.
31. A rechargeable metal-ion battery comprising: an anode, wherein
the anode comprises an electrode comprising a particulate material
as defined in claim 1 in electrical contact with a current
collector; (ii) a cathode comprising a cathode active material
capable of releasing and reabsorbing metal ions; and (iii) an
electrolyte between the anode and the cathode.
32. A particulate material comprising a plurality of composite
particles, wherein the composite particles comprise: (a) a porous
carbon framework comprising micropores and mesopores, wherein (i)
the micropores and mesopores have a total pore volume as measured
by gas adsorption of P.sub.1 cm.sup.3/g, wherein P.sub.1 has a
value of at least 0.6 and no more than 2, (ii) the volume fraction
of micropores (.phi..sub.a) is in the range from 0.5 to 0.9, based
on the total volume of micropores and mesopores; (iii) the volume
fraction of pores having a pore diameter no more than 10 nm
(.phi..sub.10) is at least 0.85, based on the total volume of
micropores and mesopores, and (iv) the porous carbon framework has
a D.sub.50 particle size of less than 20 .mu.m; (b) a plurality of
nanoscale elemental silicon domains located within the micropores
and/or mesopores of the porous carbon framework; wherein the weight
ratio of silicon to the porous carbon framework in the composite
particles is in the range from [1.times.P.sub.1 to
1.9.times.P.sub.1]:1.
33. A particulate material according to claim 32, wherein P.sub.1
has a value of at least 0.65.
34. A particulate material according to claim 32, wherein P.sub.1
has a value of no more than 1.6.
35. A particulate material according to claim 32, wherein P.sub.1
has a value of at least 0.7 and no more than 1.4.
36. A particulate material according to claim 32, wherein the
volume fraction of micropores (.phi..sub.a) is in the range from
0.5 to 0.8.
37. A particulate material according to claim 32, wherein the
volume fraction of micropores (.phi..sub.a) is in the range from
0.6 to 0.8.
38. A particulate material according to claim 32, wherein the
weight ratio of silicon to the porous carbon framework in the
composite particles is at least the value given by
[.phi..sub.b+0.75].times.P.sub.1, wherein .phi..sub.b represents
the volume fraction of mesopores, based on the total volume of
micropores and mesopores.
39. A particulate material according to claim 32, wherein the
weight ratio of silicon to the porous carbon framework in the
composite particles is at least the value given by
[.phi..sub.b+1].times.P.sub.1.
40. A particulate material according to claim 32, wherein the
weight ratio of silicon to the porous carbon framework in the
composite particles is at least the value given by
[.phi..sub.b+0.75].times.P.sub.1 and is no more than the value
given by [.phi..sub.b+1.6].times.P.sub.1.
41. A particulate material according to claim 32, wherein the
weight ratio of silicon to the porous carbon framework in the
composite particles is at least 1.25.times.P.sub.1.
42. A particulate material according to claim 32, wherein the
weight ratio of silicon to the porous carbon framework in the
composite particles no more than 1.5.times.P.sub.1.
43. A particulate material according to claim 32, wherein the
volume fraction of pores having a pore diameter of no more than 5
nm (.phi..sub.5) is at least 0.7, based on the total volume of
micropores and mesopores.
44. A particulate material according to claim 32, wherein the
porous carbon framework comprises macropores having a diameter in
the range from greater than 50 nm to 100 nm having a total volume
P.sub.2 cm.sup.3/g as measured by mercury porosimetry, wherein
P.sub.2 is no more than 0.2.times.P.sub.1.
45. A particulate material according to claim 32, wherein the
composite particles have a D.sub.50 particle diameter of at least
0.5 .mu.m and no more than 18 .mu.m, a D.sub.10 particle diameter
of at least 0.2 .mu.m and a D.sub.90 particle diameter of no more
than 40 .mu.m.
46. A particulate material according to claim 32, wherein having a
total oxygen content of no more than 10 wt %.
47. A particulate material according to claim 32, wherein having a
total oxygen content of no more than 5 wt %.
48. A particulate material according to claim 32, wherein at least
90 wt % of the silicon mass in the composite particles is located
within the internal pore volume of the porous carbon framework.
49. A composition comprising a particulate material as defined in
claim 32, and at least one other component selected from: (i) a
binder; (ii) a conductive additive; and (iii) an additional
particulate electroactive material.
50. A rechargeable metal-ion battery comprising: (i) an anode,
wherein the anode comprises an electrode comprising a particulate
material as defined in claim 32 in electrical contact with a
current collector; (ii) a cathode comprising a cathode active
material capable of releasing and reabsorbing metal ions; and (iii)
an electrolyte between the anode and the cathode.
51. A particulate material comprising a plurality of composite
particles, wherein the composite particles comprise: (a) a porous
carbon framework comprising micropores and mesopores, wherein (i)
the micropores and mesopores have a total pore volume as measured
by gas adsorption of P.sub.1 cm.sup.3/g, wherein P.sub.1 has a
value of at least 0.6 and no more than 2.0, (ii) the volume
fraction of micropores (.phi..sub.a) is in the range from 0.5 to
0.8, based on the total volume of micropores and mesopores; (iii)
the volume fraction of pores having a pore diameter no more than 10
nm (.phi..sub.10) is at least 0.75, based on the total volume of
micropores and mesopores, and (iv) the porous carbon framework has
a D.sub.50 particle size of less than 20 .mu.m; (b) a plurality of
nanoscale elemental silicon domains located within the micropores
and/or mesopores of the porous carbon framework; wherein the weight
ratio of silicon to the porous carbon framework in the composite
particles is in the range from [1.times.P.sub.1 to
1.9.times.P.sub.1]:1.
52. A particulate material according to claim 51, wherein P.sub.1
has a value of at least 0.65.
53. A particulate material according to claim 51, wherein P.sub.1
has a value of no more than 1.6.
54. A particulate material according to claim 51, wherein P.sub.1
has a value of at least 0.7 and no more than 1.4.
55. A particulate material according to claim 51, wherein the
volume fraction of micropores (.phi..sub.a) is in the range from
0.6 to 0.8.
56. A particulate material according to claim 51, wherein the
weight ratio of silicon to the porous carbon framework in the
composite particles is at least the value given by
[.phi..sub.b+0.75].times.P.sub.1, wherein .phi..sub.b represents
the volume fraction of mesopores, based on the total volume of
micropores and mesopores.
57. A particulate material according to claim 51, wherein the
weight ratio of silicon to the porous carbon framework in the
composite particles is at least the value given by
[.phi..sub.b+1].times.P.sub.1.
58. A particulate material according to claim 51, wherein the
weight ratio of silicon to the porous carbon framework in the
composite particles is no more than the value given by
[.phi..sub.b+1.5].times.P.sub.1.
59. A particulate material according to claim 51, wherein the
weight ratio of silicon to the porous carbon framework in the
composite particles is at least the value given by
[.phi..sub.b+0.75].times.P.sub.1 and is no more than the value
given by [.phi..sub.b+1.6].times.P.sub.1.
60. A particulate material according to claim 51, wherein the
weight ratio of silicon to the porous carbon framework in the
composite particles is at least 1.25.times.P.sub.1.
61. A particulate material according to claim 51, wherein the
volume fraction of pores having a pore diameter of no more than 10
nm (.phi..sub.10) is at least 0.90, based on the total volume of
micropores and mesopores.
62. A particulate material according to claim 61, wherein the
volume fraction of pores having a pore diameter of no more than 5
nm (.phi..sub.5) is at least 0.8, based on the total volume of
micropores and mesopores.
63. A particulate material according to claim 51, wherein the
volume fraction of pores having a pore diameter of no more than 5
nm (.phi..sub.5) is at least 0.7, based on the total volume of
micropores and mesopores.
64. A particulate material according to claim 51, wherein the
porous carbon framework comprises macropores having a diameter in
the range from greater than 50 nm to 100 nm having a total volume
P.sub.2 cm.sup.3/g as measured by mercury porosimetry, wherein
P.sub.2 is no more than 0.2.times.P.sub.1.
65. A particulate material according to claim 51, wherein the
composite particles have a D.sub.50 particle diameter of at least
0.5 .mu.m and no more than 18 .mu.m, a D.sub.10 particle diameter
of at least 0.2 .mu.m and a D.sub.90 particle diameter of no more
than 40 .mu.m.
66. A particulate material according to claim 51, wherein having a
total oxygen content of no more than 10 wt %.
67. A particulate material according to claim 51, wherein having a
total oxygen content of no more than 5 wt %.
68. A particulate material according to claim 51, wherein at least
90 wt % of the silicon mass in the composite particles is located
within the internal pore volume of the porous carbon framework.
69. A composition comprising a particulate material as defined in
claim 51, and at least one other component selected from: (i) a
binder; (ii) a conductive additive; and (iii) an additional
particulate electroactive material.
70. A rechargeable metal-ion battery comprising: (i) an anode,
wherein the anode comprises an electrode comprising a particulate
material as defined in claim 51 in electrical contact with a
current collector; (ii) a cathode comprising a cathode active
material capable of releasing and reabsorbing metal ions; and (iii)
an electrolyte between the anode and the cathode.
Description
BACKGROUND OF THE INVENTION
Field
This invention relates in general to electroactive materials that
are suitable for use in electrodes for rechargeable metal-ion
batteries, and more specifically to particulate materials having
high electrochemical capacities that are suitable for use as anode
active materials in rechargeable metal-ion batteries. The
particulate electroactive materials of the invention have
particular utility in hybrid anodes comprising two or more
different electroactive materials.
Technical Background
Rechargeable metal-ion batteries are widely used in portable
electronic devices such as mobile telephones and laptops and are
finding increasing application in electric or hybrid vehicles.
Rechargeable metal-ion batteries generally comprise an anode layer,
a cathode layer, an electrolyte to transport metal ions between the
anode and cathode layers, and an electrically insulating porous
separator disposed between the anode and the cathode. The cathode
typically comprises a metal current collector provided with a layer
of metal ion containing metal oxide based composite, and the anode
typically comprises a metal current collector provided with a layer
of an electroactive material, defined herein as a material which is
capable of inserting and releasing metal ions during the charging
and discharging of a battery. For the avoidance of doubt, the terms
"cathode" and "anode" are used herein in the sense that the battery
is placed across a load, such that the cathode is the positive
electrode and the anode is the negative electrode. When a metal-ion
battery is charged, metal ions are transported from the
metal-ion-containing cathode layer via the electrolyte to the anode
and are inserted into the anode material. The term "battery" is
used herein to refer both to a device containing a single anode and
a single cathode and to devices containing a plurality of anodes
and/or a plurality of cathodes.
There is interest in improving the gravimetric and/or volumetric
capacities of rechargeable metal-ion batteries. The use of
lithium-ion batteries has already provided a substantial
improvement when compared to other battery technologies, but there
remains scope for further development. To date, commercial
lithium-ion batteries have largely been limited to the use of
graphite as an anode active material. When a graphite anode is
charged, lithium intercalates between the graphite layers to form a
material with the empirical formula Li.sub.xC.sub.6 (wherein x is
greater than 0 and less than or equal to 1). Consequently, graphite
has a maximum theoretical capacity of 372 mAh/g in a lithium-ion
battery, with a practical capacity that is somewhat lower (ca. 340
to 360 mAh/g). Other materials, such as silicon, tin and germanium,
are capable of intercalating lithium with a significantly higher
capacity than graphite but have yet to find widespread commercial
use due to difficulties in maintaining sufficient capacity over
numerous charge/discharge cycles.
Silicon in particular has been identified as a promising
alternative to graphite for the manufacture of rechargeable
metal-ion batteries having high gravimetric and volumetric
capacities because of its very high capacity for lithium (see, for
example, Insertion Electrode Materials for Rechargeable Lithium
Batteries, Winter, M. et al. in Adv. Mater. 1998, 10, No. 10). At
room temperature, silicon has a theoretical maximum specific
capacity in a lithium-ion battery of about 3,600 mAh/g (based on
Li.sub.15Si.sub.4). However, the use of silicon as an anode
material is complicated by large volumetric changes on charging and
discharging.
Intercalation of lithium into bulk silicon leads to a large
increase in the volume of the silicon material, up to 400% of its
original volume when silicon is lithiated to its maximum capacity,
and repeated charge-discharge cycles cause significant mechanical
stress in the silicon material, resulting in fracturing and
delamination of the silicon anode material. Volume contraction of
silicon particles upon delithiation can result in a loss of
electrical contact between the anode material and the current
collector. A further difficulty is that the solid electrolyte
interphase (SEI) layer that forms on the silicon surface does not
have sufficient mechanical tolerance to accommodate the expansion
and contraction of the silicon. As a result, newly exposed silicon
surfaces lead to further electrolyte decomposition and increased
thickness of the SEI layer and irreversible consumption of lithium.
These failure mechanisms collectively result in an unacceptable
loss of electrochemical capacity over successive charging and
discharging cycles.
A number of approaches have been proposed to overcome the problems
associated with the volume change observed when charging
silicon-containing anodes. The most widespread approach to address
the irreversible capacity loss of silicon-containing anodes is to
use some form of finely structured silicon as the electroactive
material. It has been reported that fine silicon structures below
around 150 nm in cross-section, such as silicon films and silicon
nanoparticles are more tolerant of volume changes on charging and
discharging when compared to silicon particles in the micron size
range. However, neither of these is particularly suitable for
commercial scale applications in their unmodified form; nanoscale
particles are difficult to prepare and handle and silicon films do
not provide sufficient bulk capacity. For example, nanoscale
particles tend to form agglomerates, making it difficult to obtain
a useful dispersion of the particles within an anode material
matrix. In addition, the formation of agglomerates of nanoscale
particles results in an unacceptable capacity loss on repeated
charge-discharge cycling.
Ohara et al. (Journal of Power Sources 136 (2004) 303-306) have
described the evaporation of silicon onto a nickel foil current
collector as a thin film and the use of this structure as the anode
of a lithium-ion battery. Although this approach gives good
capacity retention, the thin film structures do not give useful
amounts of capacity per unit area, and any improvement is
eliminated when the film thickness is increased.
WO 2007/083155 discloses that improved capacity retention may be
obtained through the use of silicon particles having high aspect
ratio, i.e. the ratio of the largest dimension to the smallest
dimension of the particle.
It is also known in general terms that electroactive materials such
as silicon may be deposited within the pores of a porous carrier
material, such as an activated carbon material. These composite
materials provide some of the beneficial charge-discharge
properties of nanoscale silicon particles while avoiding the
handling difficulties of nanoparticles. For instance, Guo et al.
(Journal of Materials Chemistry A, 2013, pp. 14075-14079) discloses
a silicon-carbon composite material in which a porous carbon
substrate provides an electrically conductive framework with
silicon nanoparticles deposited within the pore structure of the
substrate with uniform distribution. SEI formation over the initial
charging cycles is confined to the remaining pore volume such that
the remaining silicon is not exposed to the electrolyte in
subsequent charging cycles. It is shown that the composite material
has improved capacity retention over multiple charging cycles,
however the initial capacity of the composite material in mAh/g is
significantly lower than for silicon nanoparticles.
JP2003100284 discloses an active material comprising a carbon-based
scaffold with small pores branching off from a few larger pores. An
electroactive material (e.g. silicon) is indiscriminately located
on the walls of both large and small pores and on the external
surface of the carbon-based scaffold.
Despite the efforts to date, there is a continued need for
improvements in the electrochemical storage capacity of lithium-ion
batteries. Although one long-term objective is to develop
electrodes containing a high proportion of silicon as the
electroactive material, another objective of battery manufacturers
is to identify ways of using small amounts of silicon to supplement
the capacity of graphite anodes. A current focus is therefore on
obtaining incremental improvements to existing metal-ion battery
technology through the use of "hybrid" electrodes comprising a
combination of graphite and Si-based electroactive materials rather
than a wholesale transition from graphite anodes to silicon
anodes.
The use of hybrid electrodes presents challenges of its own. Any
additional electroactive material must be provided in a form which
is compatible with the graphite particulate forms conventionally
used in metal-ion batteries. For example, it must be possible to
disperse the additional electroactive material throughout a matrix
of graphite particles and the particles of the additional
electroactive material must have sufficient structural integrity to
withstand compounding with graphite particles and subsequent
formation of an electrode layer, for example via steps such as
compressing, drying and calendering.
Furthermore, differences in the metallation properties of graphite
and other electroactive materials must be taken into account when
developing hybrid anodes. For example, in the lithiation of a
silicon-graphite hybrid anode in which graphite constitutes at
least 50 wt % of the electroactive material, the silicon needs to
be lithiated to its maximum capacity to gain the capacity benefit
from all the electroactive material. Whereas in a non-hybrid
silicon electrode, the silicon material would generally be limited
to ca. 25 to 60% of its maximum gravimetric capacity during charge
and discharge so as to avoid placing excessive mechanical stresses
on the silicon material and a resultant reduction in the overall
volumetric capacity of the cell, this option is not available in
hybrid electrodes. Consequently, the silicon material must be able
to withstand very high levels of mechanical stress through repeated
charge and discharge cycles.
There is therefore a need in the art for silicon-containing
electroactive materials that combine high lithiation capacity with
sufficient capacity retention and structural stability over
multiple charge-discharge cycles. In particular, materials that are
used to supplement conventional electroactive materials, such as
graphite, would need to maintain capacity and structural stability
when repeatedly lithiated to their maximum capacity.
SUMMARY OF THE DISCLOSURE
In one aspect, the disclosure provides a particulate material
comprising a plurality of composite particles, wherein the
composite particles comprise: (a) a porous carbon framework
comprising micropores and mesopores, wherein (i) the micropores and
mesopores have a total pore volume as measured by gas adsorption of
P.sub.1 cm.sup.3/g, wherein P.sub.1 has a value of at least 0.6 and
no more than 2, (ii) the volume fraction of micropores
(.phi..sub.a) is in the range from 0.5 to 0.9, based on the total
volume of micropores and mesopores; (iii) the volume fraction of
pores having a pore diameter up to 10 nm (.phi..sub.10) is at least
0.75, based on the total volume of micropores and mesopores, and
(iv) the porous carbon framework has a D.sub.50 particle size of
less than 20 .mu.m;
(b) a plurality of nanoscale domains of elemental silicon located
within the micropores and/or mesopores of the porous carbon
framework;
wherein the weight ratio of silicon to the porous carbon framework
in the composite particles is in the range from [1.times.P.sub.1 to
1.9.times.P.sub.1]:1.
Additional aspects of the disclosure relate to compositions
comprising such particulate materials, electrodes including such
particulate materials in electrical contact with a current
collector, rechargeable metal-ion batteries comprising such
electrodes, and uses of such particulate materials as anode active
materials. Other aspects will be apparent based on the detailed
description below.
DETAILED DESCRIPTION
The present invention addresses certain issues in the art by
providing a particulate material comprising a porous carbon
framework and a plurality of nanoscale domains of elemental silicon
located within the pores of the porous carbon framework. The pore
structure of the porous carbon framework and the ratio of silicon
to the available pore volume porous carbon framework are each
carefully controlled to obtain improved performance, particularly
under the demanding criteria required for hybrid electrodes.
In a first aspect, the invention provides a particulate material
comprising a plurality of composite particles, wherein the
composite particles comprise: (a) a porous carbon framework
comprising micropores and mesopores, wherein (i) the micropores and
mesopores have a total pore volume as measured by gas adsorption of
P.sub.1 cm.sup.3/g, wherein P.sub.1 has a value of at least 0.6 and
no more than 2, (ii) the volume fraction of micropores
(.phi..sub.a) is in the range from 0.5 to 0.9, based on the total
volume of micropores and mesopores; (iii) the volume fraction of
pores having a pore diameter up to 10 nm (.phi..sub.10) is at least
0.75, based on the total volume of micropores and mesopores, and
(iv) the porous carbon framework has a D.sub.50 particle size of
less than 20 .mu.m; (b) a plurality of nanoscale domains of
elemental silicon located within the micropores and/or mesopores of
the porous carbon framework; wherein the weight ratio of silicon to
the porous carbon framework in the composite particles is in the
range from [1.times.P.sub.1 to 1.9.times.P.sub.1]:1.
The invention therefore relates to a particulate material in which
the porous carbon framework comprises both micropores and mesopores
with a minimum total volume of at least 0.6 cm.sup.3/g and no more
than 2 cm.sup.3/g. The total volume of micropores and mesopores is
represented herein as P.sub.1 cm.sup.3/g. P.sub.1 itself is a
dimensionless quantity which is also used to correlate the
available pore volume to the weight ratio of silicon in the
particulate material as set out below.
The term "micropore" is used herein to refer to pores of less than
2 nm in diameter, the term "mesopore" is used herein to refer to
pores of 2-50 nm in diameter, and the term "macropore" is used to
refer to pores of greater than 50 nm and no more than 100 nm in
diameter.
The pore volume is distributed between micropores and mesopores
such that the volume fraction of micropores is in the range of 0.5
to 0.9, based on the total volume of micropores and mesopores. The
volume fraction of micropores (based on the total volume of
micropores and mesopores is represented herein by the symbol
.phi..sub.a and the volume fraction of mesopores (based on the
total volume of micropores and mesopores) is represented by the
symbol (P.sub.b, and therefore it will be understood that
.phi..sub.a+.phi..sub.b=1.
The porous carbon framework is also defined by a pore volume that
is substantially skewed towards smaller pores, such that a minimum
of 75% of the total micropore and mesopore volume is in the form of
pores having a diameter of no more than 10 nm. The volume fraction
of pores having a diameter of no more than 10 nm (based on the
total volume of micropores and mesopores) is represented herein by
the symbol .phi..sub.10, with the symbol .phi..sub.5 being used to
define the corresponding volume fraction of pores having a diameter
of no more than 5 nm.
The porous carbon framework is furthermore defined by a D.sub.50
particle size of less than 20 .mu.m
For the avoidance of doubt, P.sub.1 as used herein relates to the
pore volume of the porous carbon framework when measured in
isolation, i.e. in the absence of silicon or any other material
occupying the pores of the porous carbon framework. Similarly, the
references herein to the volume of micropores, mesopores and
macropores in the porous carbon framework, and any references to
the distribution of pore volume within the porous carbon framework,
refer to the internal pore volume of the porous carbon framework in
isolation (i.e. in the absence of any silicon or other materials
occupying the pore volume).
The weight ratio of silicon to the porous carbon framework in the
composite particles is in the range from [1.times.P.sub.1 to
1.9.times.P.sub.1]:1. The weight ratio of silicon to the porous
carbon framework is therefore proportional to the available pore
volume in the porous carbon framework, such that a weight ratio of
[1.times.P.sub.1]:1 corresponds to around 43% v/v occupancy of the
pores of the porous carbon framework by silicon, taking into
account silicon density of around 2.3 g/cm.sup.3. The upper limit
of the ratio at [1.9.times.P.sub.1]:1 corresponds to around 83% v/v
occupancy of the pores of the porous carbon framework by
silicon.
Elemental silicon is located in the micropores and/or mesopores in
the form of a plurality of nanoscale silicon domains. As used
herein, the term "nanoscale silicon domain" refers to a nanoscale
body of silicon that is located within the pores of the porous
carbon framework. The maximum dimensions of the nanoscale silicon
domains are defined by the pore diameters of the pores in which the
silicon is located.
The invention therefore relates in general terms to a particulate
material in which nanoscale domains of silicon occupy a substantial
fraction of the pore volume of a porous carbon framework in which
the pore volume is mainly distributed between small mesopores (of
no more than 10 nm diameter) and micropores. It has been found that
this particle architecture provides an electroactive material with
very high gravimetric and volumetric capacity on lithiation and
high reversible capacity retention over multiple charge-discharge
cycles.
Without being bound by theory, it is believed that locating
nanoscale silicon domains within small mesopores and/or micropores
firstly provides fine silicon structures which are able to lithiate
and delithiate without excessive structural stress. It is believed
that these very fine silicon domains have a lower resistance to
elastic deformation and higher fracture resistance than larger
silicon structures. By ensuring that a relatively high proportion
of the pore volume is occupied by silicon, the particulate material
of the invention has a high capacity. Furthermore, by locating
nanoscale silicon domains within small mesopores and/or micropores
as described above, only a small area of silicon surface is
accessible to electrolyte and SEI formation is therefore
limited.
It has been found by the present inventors that the twin objectives
of obtaining high capacity and high reversible capacity retention
depend on careful control of the distribution of pore sizes.
Although very fine silicon structures within micropores might be
expected to lithiate reversibly most effectively, it has been found
that porous carbon frameworks with an excessive micropore fraction
can accommodate relatively low amounts of silicon, such that the
volumetric capacity of the material is low. Without being bound by
theory, it is believed that deposition of silicon into very highly
microporous carbon frameworks results in the formation of silicon
structures (such as caps or walls) which block access to unoccupied
pore volume, thus limiting the silicon loading that is
achievable.
However, if silicon is deposited into carbon frameworks having a
very high degree of mesoporosity, then the silicon nanostructures
are undesirably large and the carbon wall thickness is increased.
As a result, although higher volumetric capacity can be achieved,
both the silicon nanostructures and the porous carbon framework are
subjected to excessive structural stress during
lithiation--particularly when lithiated to maximum capacity, as is
the case in hybrid anodes that also comprise graphite as an
electroactive material. This excessive structural stress can result
in fracturing of the silicon nanostructures and the porous carbon
framework. Additional exposure of silicon from the fracture
surfaces to the electrolyte in subsequent charge-discharge cycles
then means that SEI formation can become a significant failure
mechanism leading to capacity loss. By controlling the relative
volume fractions of micropores and mesopores and by ensuring that
the mesopore volume is largely confined to pores below 10 nm, the
particulate material of the invention avoids these failure
mechanisms and maintains high reversible capacity over multiple
charge-discharge cycles while also accommodating a large relative
proportion of silicon within the pore volume of the porous carbon
framework. This stands in clear contrast to the excessive SEI
formation that characterizes the material disclosed by Guo, for
example (see above).
Although lithiation of the silicon may result in a degree of
external expansion of the entire composite material, the careful
control of the micropore and mesopore volume fractions and the size
distribution of the mesopore volume fraction towards smaller pore
diameters ensures that the particulate material is able to deform
reversibly without fracturing over multiple charge-discharge
cycles. Stress on the carbon framework and the silicon material is
therefore controlled at a level that is tolerated over large
numbers of charge-discharge cycles without substantial loss of
capacity.
As a result of the unique particle architecture of the inventive
composite material, the silicon in the particulate material of the
invention has electrochemical performance that is comparable to
that of fine silicon nanoparticles but without the disadvantages of
excessive SEI formation and poor dispersibility that make discrete
silicon nanoparticles non-viable as an electrode material for
commercial use. The relatively high volumetric content of silicon
of the particulate material makes it particularly suitable for use
as a component of hybrid anodes.
The porous carbon framework suitably comprises a
three-dimensionally interconnected open pore network comprising a
combination of micropores and/or mesopores and optionally a minor
volume of macropores. The porous carbon framework is characterized
by a high pore volume in the form of micropores and/or mesopores.
The total volume of micropores and mesopores (i.e. the total pore
volume in the range of 0 to 50 nm) is referred to herein as P.sub.1
cm.sup.3/g, wherein P.sub.1 represents a dimensionless natural
number having a value of at least 0.6 and no more than 2. As set
out above, the value of P.sub.1 is also used to correlate the
available pore volume in the porous carbon framework to the weight
ratio of silicon to the porous carbon framework as set out
above.
Preferably, the value of P.sub.1 is at least 0.65, more preferably
at least 0.7, more preferably at least 0.75, more preferably at
least 0.8, more preferably at least 0.85, more preferably at least
0.9, more preferably at least 0.95, and most preferably at least 1.
Optionally, the total volume of micropores and mesopores may be
greater than 1 cm.sup.3/g, for instance, P.sub.1 may be at least
1.05, or at least 1.1, or at least 1.15, or at least 1.2.
The use of a high porosity carbon framework is advantageous since
it allows a larger amount of silicon to be accommodated within the
pore structure, and it has been found that high porosity carbon
frameworks in which the pore volume is predominantly in the form of
micropores and smaller mesopores have sufficient strength to
accommodate the volumetric expansion of the silicon without
fracturing or otherwise degrading the porous carbon framework.
The internal pore volume of the porous carbon framework is capped
at a value at which increasing fragility of the porous carbon
framework outweighs the advantage of increased pore volume
accommodating a larger amount of silicon. In general, the value of
P.sub.1 is no more than 2. However, more preferably, the value of
P.sub.1 may be no more than 1.8, more preferably no more than 1.6,
more preferably no more than 1.5, more preferably no more than 1.4,
more preferably no more than 1.3, more preferably no more than
1.2.
In accordance with certain embodiments of the invention, the value
of P.sub.1 may be, for instance, in the range from 0.65 to 2 (i.e.
a total volume of micropores and mesopores of 0.65 to 2
cm.sup.3/g), or in the range from 0.65 to 1.8, or in the range from
0.7 to 1.8, or in the range from 0.75 to 1.8, or in the range from
0.8 to 1.8, or in the range from 0.85 to 1.8, or in the range from
0.9 to 1.8, or in the range from 0.65 to 1.7, or in the range from
0.7 to 1.7, or in the range from 0.75 to 1.7, or in the range from
0.8 to 1.7, or in the range from 0.85 to 1.7, or in the range from
0.9 to 1.7, or in the range from 0.95 to 1.7, or in the range from
0.7 to 1.6, or in the range from 0.75 to 1.6, or in the range from
0.8 to 1.6, or in the range from 0.85 to 1.6, or in the range from
0.9 to 1.6, or in the range from 0.95 to 1.6, or in the range from
1 to 1.6, or in the range from 0.75 to 1.5, or in the range from
0.8 to 1.5, or in the range from 0.85 to 1.5, or in the range from
0.9 to 1.5, or in the range from 0.95 to 1.5, or in the range from
1 to 1.5, or in the range from 0.8 to 1.4, or in the range from
0.85 to 1.4, or in the range from 0.9 to 1.4, or in the range from
0.95 to 1.4, or in the range from 1 to 1.4.
The volume fraction of micropores (.phi..sub.a) is preferably in
the range from 0.5 to 0.85, more preferably in the range from 0.5
to 0.8, more preferably in the range from 0.55 to 0.8, more
preferably in the range from 0.6 to 0.8, to take particular
advantage of the high capacity retention of very fine silicon
nanostructures located within micropores.
As discussed above, the pore volume is substantially skewed towards
smaller pores, such that a minimum of 75% of the total micropore
and mesopore volume of the porous carbon framework is in the form
of pores having a diameter of no more than 10 nm. More preferably,
.phi..sub.10 is at least 0.8, more preferably at least 0.85, more
preferably at least 0.9.
Preferably, .phi..sub.5 is at least 0.7, more preferably at least
0.75, more preferably at least 0.8, more preferably at least 0.85,
based on the total volume of micropores and mesopores. Thus, at
least 75% of the total micropore and mesopore volume of the porous
carbon framework is preferably in the form of pores having a
diameter of no more than 10 nm, and more preferably no more than 5
nm.
A fraction of pores having diameters in the larger mesopore range
may be advantageous to facilitate electrolyte access to the silicon
domains. Therefore, pores having a diameter in the range of 10 to
50 nm (i.e. larger mesopores) may optionally constitute up to 1%,
up to 2%, up to 5%, or up to 10% of the total micropore and
mesopore volume of the porous carbon framework.
The pore size distribution of the porous carbon framework may be
monomodal, bimodal or multimodal. As used herein, the term "pore
size distribution" relates to the distribution of pore size
relative to the cumulative total internal pore volume of the porous
carbon framework, not only of micropores and mesopores but also of
any macropores present. A bimodal or multimodal pore size
distribution may be preferred since close proximity between the
smallest pores and pores of larger diameter provides the advantage
of efficient ionic transport through the porous network to the
silicon. Accordingly, the particulate material has high ionic
diffusivity and therefore improved rate performance.
Suitably, a bimodal or multimodal pore size distribution includes a
peak pore size in the micropore range and a peak pore size in the
mesopore size range which differ from one another by a factor of
from 5 to 20, more preferably by a factor of about 10. For
instance, the porous carbon framework may have a bimodal pore size
distribution including a peak at a pore size of 1.5 nm and a peak
at a pore size of 7.5 nm.
The total volume of micropores and mesopores and the pore size
distribution of micropores and mesopores are determined using
nitrogen gas adsorption at 77 K using quenched solid density
functional theory (QSDFT) in accordance with standard methodology
as set out in ISO 15901-2 and ISO 15901-3. Nitrogen gas adsorption
is a technique that characterizes the porosity and pore diameter
distributions of a material by allowing a gas to condense in the
pores of a solid. As pressure increases, the gas condenses first in
the pores of smallest diameter and the pressure is increased until
a saturation point is reached at which all of the pores are filled
with liquid. The nitrogen gas pressure is then reduced
incrementally, to allow the liquid to evaporate from the system.
Analysis of the adsorption and desorption isotherms, and the
hysteresis between them, allows the pore volume and pore size
distribution to be determined. Suitable instruments for the
measurement of pore volume and pore size distributions by nitrogen
gas adsorption include the TriStar II and TriStar II Plus porosity
analyzers, which are available from Micromeritics Instrument
Corporation, USA.
Nitrogen gas adsorption is effective for the measurement of pore
volume and pore size distributions for pores having a diameter up
to 50 nm, but is less reliable for pores of much larger diameter.
For the purposes of the present invention, nitrogen adsorption is
therefore used to determine pore volumes and pore size
distributions only for pores having a diameter up to and including
50 nm. As set out above, the value of P.sub.1 is determined by
taking into account only pores of diameter up to and including 50
nm (i.e. only micropores and mesopores), and the values of
.phi..sub.a, .phi..sub.b, .phi..sub.10, and .phi..sub.5 are
likewise determined relative to the total volume of micropores and
mesopores only.
In view of the limitations of available analytical techniques it is
not possible to measure pore volumes and pore size distributions
across the entire range of micropores, mesopores and macropores
using a single technique. In the case that the porous carbon
framework comprises macropores, the volume of pores in the range of
greater than 50 nm and up to 100 nm is identified herein with the
value of P.sub.2 cm.sup.3/g and is measured by mercury porosimetry.
As set out above, the value of P.sub.2 relates to the pore volume
of the porous carbon framework when measured in isolation, i.e. in
the absence of silicon or any other material occupying the pores of
the porous carbon framework.
For the avoidance of doubt, the value of P.sub.2 takes into account
only pores having a diameter of from greater than 50 nm up to and
including 100 nm, i.e. it includes only the volume of macropores up
to 100 nm in diameter. Any pore volume measured by mercury
porosimetry at pore sizes of 50 nm or below is disregarded for the
purposes of determining the value of P.sub.2 (as set out above,
nitrogen adsorption is used to characterize the mesopores and
micropores). Pore volume measured by mercury porosimetry above 100
nm is assumed for the purposes of the invention to be
inter-particle porosity and is also not take into account in the
measurement of macropores or when determining the value of
P.sub.2.
Mercury porosimetry is a technique that characterizes the porosity
and pore diameter distributions of a material by applying varying
levels of pressure to a sample of the material immersed in mercury.
The pressure required to intrude mercury into the pores of the
sample is inversely proportional to the size of the pores. Values
obtained by mercury porosimetry as reported herein are obtained in
accordance with ASTM UPP578-11, with the surface tension .gamma.
taken to be 480 mN/m and the contact angle .phi. taken to be
140.degree. for mercury at room temperature. The density of mercury
is taken to be 13.5462 g/cm.sup.3 at room temperature. A number of
high precision mercury porosimetry instruments are commercially
available, such as the AutoPore IV series of automated mercury
porosimeters available from Micromeritics Instrument Corporation,
USA. For a complete review of mercury porosimetry reference may be
made to P. A. Webb and C. Orr in "Analytical Methods in Fine
Particle Technology, 1997, Micromeritics Instrument Corporation,
ISBN 0-9656783-0.
The volume of macropores (and therefore the value of P.sub.2) is
preferably small as compared to the volume of micropores and
mesopores (and therefore the value of P.sub.1). While a small
fraction of macropores may be useful to facilitate electrolyte
access into the pore network, the advantages of the invention are
obtained substantially by accommodating silicon in micropores and
smaller mesopores.
Thus, in accordance with certain embodiments of the invention the
total volume of macropores in the porous carbon framework is
P.sub.2 cm.sup.3/g as measured by mercury porosimetry, wherein
P.sub.2 preferably has a value of no more than 0.2.times.P.sub.1,
or no more than 0.1.times.P.sub.1, or no more than
0.05.times.P.sub.1, or no more than 0.02.times.P.sub.1, or no more
than 0.01.times.P.sub.1, or no more than 0.005.times.P.sub.1.
P.sub.2 preferably has a value of no more than 0.3, or no more than
0.25, or no more than 0.20, or no more than 0.15, or no more than
0.1, or no more than 0.05. As discussed above in relation to larger
mesopores, a small pore volume fraction in the macropore range may
be advantageous to facilitate electrolyte access to the
silicon.
The open pore network optionally includes a hierarchical pore
structure, i.e. a pore structure in which there is a degree of
ordering of pore sizes, with smaller pores branching from larger
pores.
It will be appreciated that intrusion techniques such as gas
adsorption and mercury porosimetry are effective only to determine
the pore volume of pores that are accessible to nitrogen or to
mercury from the exterior of the porous carbon framework. Porosity
values (P.sub.1 and P.sub.2) as specified herein shall be
understood as referring to the volume of open pores, i.e. pores
that are accessible to a fluid from the exterior of the porous
carbon framework. Fully enclosed pores which cannot be identified
by nitrogen adsorption or mercury porosimetry shall not be taken
into account herein when specifying porosity values. Likewise, any
pore volume located in pores that are so small as to be below the
limit of detection by nitrogen adsorption is not taken into account
for determining the value of P.sub.1.
The porous carbon framework may comprise crystalline carbon or
amorphous carbon, or a mixture of amorphous and crystalline carbon.
The porous carbon framework may be either a hard carbon or soft
carbon framework and may suitably be obtained by known procedures
involving the pyrolysis of polymers or organic matter.
As used herein, the term "hard carbon" refers to a disordered
carbon matrix in which carbon atoms are found predominantly in the
sp.sup.2 hybridized state (trigonal bonds) in nanoscale
polyaromatic domains. The polyaromatic domains are cross-linked
with a chemical bond, e.g. a C--O--C bond. Due to the chemical
cross-linking between the polyaromatic domains, hard carbons cannot
be converted to graphite at high temperatures. Hard carbons have
graphite-like character as evidenced by the large G-band
(.about.1600 cm-1) in the Raman spectrum. However, the carbon is
not fully graphitic as evidenced by the significant D-band
(.about.1350 cm-1) in the Raman spectrum.
As used herein, the term "soft carbon" also refers to a disordered
carbon matrix in which carbon atoms are found predominantly in the
sp.sup.2 hybridized state (trigonal bonds) in polyaromatic domains
having dimensions in the range of 5-200 nm. In contrast to hard
carbons, the polyaromatic domains in soft carbons are associated by
intermolecular forces but are not cross-linked with a chemical
bond. This means that they will graphitize at high temperature. The
porous carbon framework preferably comprises at least 50% sp.sup.2
hybridized carbon as measured by XPS. For example, the porous
carbon framework may suitably comprise from 50% to 98% sp.sup.2
hybridized carbon, from 55% to 95% sp.sup.2 hybridized carbon, from
60% to 90% sp.sup.2 hybridized carbon, or from 70% to 85% sp.sup.2
hybridized carbon.
A variety of different polymeric materials may be used to prepare
suitable porous carbon frameworks. Examples of polymeric materials
which form porous carbon frameworks on pyrolysis include phenolic
resins, novolac resins, pitch, melamines, polyacrylates,
polystyrenes, polyvinylalcohol (PVA), polyvinylpyrrolidone (PVP),
and various copolymers comprising monomer units of acrylates,
styrenes, .alpha.-olefins, vinyl pyrrolidone and other
ethylenically unsaturated monomers. A variety of different hard
carbon materials are available in the art depending on the starting
material and the conditions of the pyrolysis process.
The porous carbon framework may undergo a chemical or gaseous
activation process to increase the volume of mesopores and
micropores. A suitable activation process comprises contacting
pyrolysed carbon with one or more of oxygen, steam, CO, CO.sub.2
and KOH at a temperature in the range from 600 to 1000.degree.
C.
Mesopores can also be obtained by known templating processes, using
extractable pore formers such as MgO and other colloidal or polymer
templates which can be removed by thermal or chemical means post
pyrolysis or activation.
The porous carbon framework may have a D.sub.50 particle diameter
in the range from 0.5 to 20 .mu.m. Preferably, the D.sub.50
particle diameter is at least 1 .mu.m, more preferably at least 2
.mu.m, for example at least 3 .mu.m, or at least 4 .mu.m, or at
least 5 .mu.m. Preferably, the D.sub.50 particle diameter of the
particulate material is no more than 18 .mu.m, more preferably no
more than 16 .mu.m, more preferably no more than 14 .mu.m, more
preferably no more than 12 .mu.m, more preferably no more than 10
.mu.m, for example no more than 9 .mu.m, or no more than 8
.mu.m.
For instance, the porous carbon framework may have a D.sub.50
particle diameter in the range from 1 to 12 .mu.m, or from 1 to 10
.mu.m, or from 2 to 10 .mu.m, or from 3 to 10 .mu.m, or from 3 to 8
.mu.m.
The amount of silicon in the porous carbon framework is correlated
to the available pore volume by the requirement that the weight
ratio of silicon to the porous carbon framework in the composite
particles is in the range from [1.times.P.sub.1 to
1.9.times.P.sub.1]:1. This relationship takes into account the
density of silicon and the pore volume of the porous carbon
framework to define a weight ratio of silicon at which the internal
pore volume of the porous carbon framework (P.sub.1 cm.sup.3/g) is
around 43% to 82% v/v occupied by silicon (in the uncharged
state).
Preferably, the weight ratio of silicon to the porous carbon
framework is at least 1.1.times.P.sub.1, more preferably at least
1.15.times.P.sub.1, more preferably at least 1.2.times.P.sub.1,
more preferably at least 1.25.times.P.sub.1, more preferably at
least 1.3.times.P.sub.1, more preferably at least
1.35.times.P.sub.1, more preferably at least 1.4.times.P.sub.1.
The minimum weight ratio of silicon to the porous carbon framework
is correlated to the total micropore and mesopore volume by the
requirement that the weight ratio of silicon to the porous carbon
framework in the composite particles is at least 1.times.P.sub.1.
More preferably, the weight ratio of silicon to the porous carbon
framework has at least the value given by
[.phi..sub.b+0.75].times.P.sub.1, more preferably at least the
value given by [.phi..sub.b+0.8].times.P.sub.1, more preferably at
least the value given by [.phi..sub.b+0.9].times.P.sub.1, more
preferably at least the value given by
[.phi..sub.b+1].times.P.sub.1, more preferably at least the value
given by [.phi..sub.b+1.1].times.P.sub.1 (with the proviso that
said value is at least 1.times.P.sub.1).
Thus, in the case that the mesopore fraction (.phi..sub.b) has a
higher value, the minimum amount of silicon in the composite
particles is also higher. This correlation between mesopore
fraction and the minimum weight ratio of silicon to the porous
carbon framework ensures that porous carbon frameworks having
higher mesopore fractions are occupied by silicon to a higher
extent, thus improving the volumetric capacity of the particulate
material. Ensuring that porous carbon frameworks having higher
mesopore fractions have a higher minimum silicon loading also
reduces the possibility that larger micropores will be partially
occupied by silicon, thus reducing the silicon surface area that is
exposed to the electrolyte and thereby limiting undesirable SEI
formation.
The maximum weight ratio of silicon to the porous carbon framework
is also correlated to the total pore volume by the requirement that
the weight ratio of silicon to the porous carbon framework in the
composite particles is no more than 1.9.times.P.sub.1. More
preferably, the weight ratio of silicon to the porous carbon
framework is no more than the value given by
[.phi..sub.b+1.6].times.P.sub.1, more preferably no more than the
value given by [.phi..sub.b+1.5].times.P.sub.1 (with the proviso
that said value is no more than 1.9.times.P.sub.1).
The correlation between mesopore fraction and the maximum weight
ratio of porous carbon framework ensures that that porous carbon
frameworks having higher micropore fractions are not excessively
filled by silicon. As set out above, it may be more difficult to
achieve very high ratios of silicon in the event that the porous
carbon framework is more highly microporous due to the potential
for walls or caps to form which enclose occupied pore volume. In
addition, in the case that the porous carbon framework is more
highly microporous, diffusion of lithium through very fine silicon
structures becomes rate limited, reducing the rate capacity of the
particulate material. Accordingly, controlling the upper limit of
the silicon ratio ensures a degree of electrolyte access to the
internal pore volume of the porous carbon framework, facilitating
transport of lithium ions to the silicon domains.
Preferably the silicon mass in the composite particles is located
substantially or entirely within the pores of the porous carbon
framework in the form of the nanoscale silicon domains that are
described above. For example, it is preferred that at least 90 wt
%, more preferably at least 95 wt %, more preferably at least 98 wt
%, more preferably at least 99 wt % of the silicon mass in the
composite particles is located within the internal pore volume of
the porous carbon framework such that there is no or very little
silicon located on the external surface of the composite
particles.
Preferably, the volume of micropores and mesopores in the composite
particles (i.e. in the presence of the silicon), as measured by
nitrogen gas adsorption, is no more than 0.15.times.P.sub.1, or no
more than 0.10.times.P.sub.1, or no more than 0.05.times.P.sub.1,
or no more than 0.02.times.P.sub.1.
The weight ratio of silicon to the porous carbon framework can be
determined by elemental analysis. Elemental analysis is used to
determine the weight percentages of both silicon and carbon in the
composite particles. Optionally, the amounts of hydrogen, nitrogen
and oxygen may also be determined by elemental analysis.
Preferably, elemental analysis is also used to determine the weight
percentage of carbon (and optionally hydrogen, nitrogen and oxygen)
in the porous carbon framework alone. Determining the weight
percentage of carbon in the in the porous carbon framework alone
takes account of the possibility that the porous carbon framework
contains a minor amount of heteroatoms within its molecular
framework. Both measurements taken together allow the weight
percentage of silicon relative to the entire porous carbon
framework to be determined reliably.
The silicon content is determined by ICP-OES (Inductively coupled
plasma-optical emission spectrometry). A number of ICP-OES
instruments are commercially available, such as the iCAP.RTM. 7000
series of ICP-OES analyzers available from ThermoFisher Scientific.
The carbon content of the composite particles and of the porous
carbon framework alone (as well as the hydrogen, nitrogen and
oxygen content if required) are determined by IR absorption-based
elemental analysis. Suitable instruments for determining carbon,
hydrogen, nitrogen and oxygen content include the TruSpec.RTM.
Micro elemental analyser, the LECO model CS844 (carbon) and the
LECO model ONH836 (hydrogen, nitrogen and oxygen), all available
from Leco Corporation.
In certain aspects, the composite particles described herein are
formed chiefly of silicon, carbon and oxygen (e.g., individually in
the amounts as described above). In certain embodiments, the sum of
the amount of silicon and carbon (measured as described above) of
the composite particles is at least 80%, e.g., at least 85 wt %, at
least 90 wt %, or at least 95 wt %. For example, in various
embodiments, the sum of the amount of silicon and carbon is in the
range of 80-98 wt %, or 85-98 wt %, or 90-98 wt %, or 95-98 wt %.
In certain embodiments, the sum of the amount of silicon, carbon
and oxygen (measured as described above) is at least 90 wt %, e.g.,
at least 95 wt %, at least 97 wt %, or at least 98 wt %. For
example in various embodiments, the sum of the amount of silicon,
carbon and oxygen is in the range of 90-105 wt %, or 90-100 wt %,
or 90-99 wt %, or 95-105 wt %, or 95-100 wt %, or 97-105 wt %, or
97-100 wt %, or 98-105 wt %, or 98-100 wt %. The person of ordinary
skill in the art will appreciate that the different measurement
techniques used may result in a sum of the as-measured silicon,
carbon, and oxygen amounts that are slightly above 100%. And as the
person of ordinary skill in the art will appreciate, many of the
elemental analysis techniques described herein are destructive,
such that different test samples from a single composition must be
tested to provide, e.g., a measurement of silicon weight percent
and a measurement of carbon weight percent.
The composite particles preferably have a low total oxygen content.
Oxygen may be present in the composite particles for instance as
part of the porous carbon framework or as an oxide layer on any
exposed silicon surfaces. Preferably, the total oxygen content of
the composite particles is less than 15 wt %, more preferably less
than 10 wt %, more preferably less than 5 wt %, for example less
than 2 wt %, or less than 1 wt %, or less than 0.5 wt %.
The silicon may optionally comprise a minor amount of one or more
dopants. Suitable dopants include boron and phosphorus, other
n-type or p-type dopants, nitrogen, or germanium. Preferably, the
dopants are present in a total amount of no more than 2 wt % based
on the total amount of silicon and the dopant(s).
For the avoidance of doubt, the term "particle diameter" as used
herein refers to the equivalent spherical diameter (esd), i.e. the
diameter of a sphere having the same volume as a given particle,
wherein the particle volume is understood to include the volume of
any intra-particle pores. The terms "D.sub.50" and "D.sub.50
particle diameter" as used herein refer to the volume-based median
particle diameter, i.e. the diameter below which 50% by volume of
the particle population is found. The terms "D.sub.10" and
"D.sub.10 particle diameter" as used herein refer to the 10th
percentile volume-based median particle diameter, i.e. the diameter
below which 10% by volume of the particle population is found. The
terms "D.sub.90" and "D.sub.90 particle diameter" as used herein
refer to the 90th percentile volume-based median particle diameter,
i.e. the diameter below which 90% by volume of the particle
population is found.
Particle diameters and particle size distributions are determined
by routine laser diffraction techniques in accordance with ISO
13320:2009. Laser diffraction relies on the principle that a
particle will scatter light at an angle that varies depending on
the size the particle and a collection of particles will produce a
pattern of scattered light defined by intensity and angle that can
be related to a particle size distribution. A number of laser
diffraction instruments are commercially available for the rapid
and reliable determination of particle size distributions. Unless
stated otherwise, particle size distribution measurements as
specified or reported herein are as measured by the conventional
Malvern Mastersizer.TM. 3000 particle size analyzer from Malvern
Instruments. The Malvern Mastersizer.TM. 3000 particle size
analyzer operates by projecting a helium-neon gas laser beam
through a transparent cell containing the particles of interest
suspended in an aqueous solution. Light rays which strike the
particles are scattered through angles which are inversely
proportional to the particle size and a photodetector array
measures the intensity of light at several predetermined angles and
the measured intensities at different angles are processed by a
computer using standard theoretical principles to determine the
particle size distribution. Laser diffraction values as reported
herein are obtained using a wet dispersion of the particles in
distilled water. The particle refractive index is taken to be 3.50
and the dispersant index is taken to be 1.330. Particle size
distributions are calculated using the Mie scattering model.
The composite particles may have a D.sub.50 particle diameter in
the range from 0.5 to 20 .mu.m. Preferably, the D.sub.50 particle
diameter is at least 1 .mu.m, more preferably at least 2 .mu.m, for
example at least 3 .mu.m, or at least 4 .mu.m, or at least 5 .mu.m.
Preferably, the D.sub.50 particle diameter of the particulate
material is no more than 18 .mu.m, more preferably no more than 16
.mu.m, more preferably no more than 14 .mu.m, more preferably no
more than 12 .mu.m, more preferably no more than 10 .mu.m, for
example no more than 9 .mu.m, or no more than 8 .mu.m.
For instance, the composite particles may have a D.sub.50 particle
diameter in the range from 1 to 12 .mu.m, or from 1 to 10 .mu.m, or
from 2 to 10 .mu.m, or from 3 to 10 .mu.m, or from 3 to 8 .mu.m.
Particles within these size ranges and having porosity and a pore
diameter distribution as set out herein are ideally suited for use
in hybrid anodes for metal-ion batteries, due to their
dispersibility in slurries, their structural robustness, their
capacity retention over repeated charge-discharge cycles, and their
ability to occupy the interparticle void spaces between
conventional graphite particles used in the electrode of metal-ion
batteries.
The D.sub.10 particle diameter of the composite particles is
preferably at least 0.2 .mu.m, or at least 0.5 .mu.m, or at least
0.8 .mu.m, or at least 1 .mu.m, or at least 1.5 .mu.m, or at least
2 .mu.m. By maintaining the D.sub.10 particle diameter at 0.2 .mu.m
or more, the potential for undesirable agglomeration of sub-micron
sized particles is reduced, resulting in improved dispersibility of
the particulate material and improved capacity retention.
The D.sub.90 particle diameter of the composite particles is
preferably no more than 40 .mu.m, or no more than 30 .mu.m, or no
more than 20 .mu.m, or no more than 15 .mu.m, or no more than 12
.mu.m, or no more than 10 .mu.m. The presence of very large
particles results in non-uniform forming packing of the particles
in electrode active layers, thus disrupting the formation of dense
electrode layers, particularly electrode layers having a thickness
in the conventional range from 20 to 50 .mu.m. Therefore, it is
preferred that the D.sub.90 particle diameter is no more than 20
.mu.m, and more preferably lower still.
The composite particles preferably have a narrow size distribution
span. For instance, the particle size distribution span (defined as
(D.sub.90-D.sub.10)/D.sub.50) is preferably 5 or less, more
preferably 4 or less, more preferably 3 or less, more preferably 2
or less, and most preferably 1.5 or less. By maintaining a narrow
size distribution span, efficient packing of the particles into
dense electrode layers is more readily achievable.
The composite particles may be spheroidal in shape. Spheroidal
particles as defined herein may include both spherical and
ellipsoidal particles and the shape of the composite particles of
the invention may suitably be defined by reference to the
sphericity and the aspect ratio of the particles of the invention.
Spheroidal particles are found to be particularly well-suited to
dispersion in slurries without the formation of agglomerates. In
addition, the use of porous spheroidal particles is surprisingly
found to provide a further improvement in strength when compared to
porous particles and porous particle fragments of irregular
morphology.
The sphericity of an object is conventionally defined as the ratio
of the surface area of a sphere to the surface area of the object,
wherein the object and the sphere have identical volume. However,
in practice it is difficult to measure the surface area and volume
of individual particles at the micron scale. However, it is
possible to obtain highly accurate two-dimensional projections of
micron scale particles by scanning electron microscopy (SEM) and by
dynamic image analysis, in which a digital camera is used to record
the shadow projected by a particle. The term "sphericity" as used
herein shall be understood as the ratio of the area of the particle
projection to the area of a circle, wherein the particle projection
and circle have identical circumference. Thus, for an individual
particle, the sphericity S may be defined as:
.pi. ##EQU00001## wherein A.sub.m is the measured area of the
particle projection and C.sub.m is the measured circumference of
the particle projection. The average sphericity S.sub.av of a
population of particles as used herein is defined as:
.times..times..pi. ##EQU00002## wherein n represents the number of
particles in the population.
As used herein, the term "spheroidal" as applied to the composite
particles of the invention shall be understood to refer to a
material having an average sphericity of at least 0.70. Preferably,
the porous spheroidal particles of the invention have an average
sphericity of at least 0.85, more preferably at least 0.90, more
preferably at least 0.92, more preferably at least 0.93, more
preferably at least 0.94, more preferably at least 0.95.
Optionally, the porous spheroidal particles may have an average
sphericity of at least 0.96, or at least 0.97, or at least 0.98, or
at least 0.99.
It will be understood that the circumference and area of a
two-dimensional particle projection will depend on the orientation
of the particle in the case of any particle which is not perfectly
spheroidal. However, the effect of particle orientation may be
offset by reporting sphericity and aspect ratios as average values
obtained from a plurality of particles having random orientation. A
number of SEM and dynamic image analysis instruments are
commercially available, allowing the sphericity and aspect ratio of
a particulate material to be determined rapidly and reliably.
Unless stated otherwise, sphericity values as specified or reported
herein are as measured by a CamSizer XT particle analyzer from
Retsch Technology GmbH. The CamSizer XT is a dynamic image analysis
instrument which is capable of obtaining highly accurate
distributions of the size and shape for particulate materials in
sample volumes of from 100 mg to 100 g, allowing properties such as
average sphericity and aspect ratios to be calculated directly by
the instrument.
The composite particles preferably have a BET surface area of no
more than 30 m.sup.2/g, or no more than 25 m.sup.2/g, or no more
than 20 m.sup.2/g, or no more than 15 m.sup.2/g, or no more than 10
m.sup.2/g. The term "BET surface area" as used herein should be
taken to refer to the surface area per unit mass calculated from a
measurement of the physical adsorption of gas molecules on a solid
surface, using the Brunauer-Emmett-Teller theory, in accordance
with ISO 9277. In general, a low BET surface area is preferred in
order to minimize the formation of solid electrolyte interphase
(SEI) layers at the surface of the composite particles during the
first charge-discharge cycle of an anode comprising the particulate
material of the invention. However, a BET surface area which is
excessively low results in unacceptably low charging rate and
capacity due to the inaccessibility of the bulk of the
electroactive material to metal ions in the surrounding
electrolyte. For instance, the BET surface area is preferably at
least 0.1 m.sup.2/g, or at least 1 m.sup.2/g, or at least 2
m.sup.2/g, or at least 5 m.sup.2/g. For instance, the BET surface
area may be in the range from 1 to 25 m.sup.2/g, more preferably in
the range from 2 to 15 m.sup.2/g.
The particulate material of the invention typically has a specific
charge capacity on first lithiation of 1200 to 2340 mAh/g.
Preferably the particulate material of the invention has a specific
charge capacity on first lithiation of at least 1400 mAh/g.
The composite particles of the invention are suitably prepared via
the chemical vapor infiltration (CVI) of a silicon-containing
precursor into the pore structure of the porous carbon framework.
As used herein, CVI refers to processes in which a gaseous
silicon-containing precursor is thermally decomposed on a surface
to form elemental silicon at the surface and gaseous
by-products.
Suitable gaseous silicon-containing precursors include silane
(SiH.sub.4), silane derivatives (e.g. disilane, trisilane and
tetrasilane), and trichlorosilane (SiHCl.sub.3). The
silicon-containing precursors may be used either in pure form or
more usually as a diluted mixture with an inert carrier gas, such
as nitrogen or argon. For instance, the silicon-containing
precursor may be used in an amount in the range from 0.5-20 vol %,
or 1-10 vol %, or 1-5 vol % based on the total volume of the
silicon-containing precursor and the inert carrier gas. The CVI
process is suitably carried out at low partial pressure of silicon
precursor with total pressure of 101.3 kPa (i.e. 1 atm), the
remaining partial pressure made up to atmospheric pressure using an
inert padding gas such as hydrogen, nitrogen or argon. Deposition
temperatures ranging from 400-700.degree. C. are used, for example
from 450-550.degree. C., or 450-500.degree. C. The CVI process may
suitably be performed in a fixed bed reactor, fluidized bed
reactor, or rotary kiln.
As an example of a fixed-bed reactor method, 1.8 g of a particulate
porous framework was placed on a stainless-steel plate at a
constant thickness of 1 mm along its length. The plate was then
placed inside a stainless-steel tube of outer diameter 60 mm with
gas inlet and outlet lines located in the hot zone of a retort
furnace. The furnace tube was purged with nitrogen gas for 30
minutes at room temperature, then the sample temperature was
increased to 450-500.degree. C. The nitrogen gas flow-rate is
adjusted to ensure a gas residence time of at least 90 seconds in
the furnace tube and maintained at that rate for 30 minutes.
Then, the gas supply is switched from nitrogen to a mixture of
monosilane in nitrogen at 1.25 vol. % concentration. Dosing of
monosilane is performed over a 5-hour period with a reactor
pressure maintained at 101.3 kPa (1 atm). After dosing has finished
the gas flow rate is kept constant whilst the silane is purged from
the furnace using nitrogen. The furnace is purged for 30 minutes
under nitrogen before being cooled down to room temperature over
several hours. The atmosphere is then switched over to air
gradually over a period of two hours by switching the gas flow from
nitrogen to air from a compressed air supply.
The particulate material of the invention may optionally include a
conductive carbon coating. Suitably a conductive carbon coating may
be obtained by a chemical vapor deposition (CVD) method. CVD is a
well-known methodology in the art and comprises the thermal
decomposition of a volatile carbon-containing gas (e.g. ethylene)
onto the surface of the particulate material. Alternatively, the
carbon coating may be formed by depositing a solution of a
carbon-containing compound onto the surface of the particulate
material followed by pyrolysis. The conductive carbon coating is
sufficiently permeable to allow lithium access to the interior of
the composite particles without excessive resistance, so as not to
reduce the rate performance of the composite particles. For
instance, the thickness of the carbon coating may suitably be in
the range from 2 to 30 nm. Optionally, the carbon coating may be
porous and/or may only cover partially the surface of the composite
particles.
A carbon coating has the advantages that it further reduces the BET
surface area of the particulate material by smoothing any surface
defects and by filling any remaining surface microporosity, thereby
further reducing first cycle loss. In addition, a carbon coating
improves the conductivity of the surface of the composite
particles, reducing the need for conductive additives in the
electrode composition, and also creates an improved surface for the
formation of a stable SEI layer, resulting in improved capacity
retention on cycling.
In accordance with the first aspect of the invention, there is
further provided particulate materials according to the following
aspects 1a-1bb.
Aspect 1a: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.7-1.4; (ii) .phi..sub.a is in the
range from 0.5 to 0.8; (iii) .phi..sub.10 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 1 to 18 .mu.m.
Aspect 1 b: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.7-1.4; (ii) .phi..sub.a is in the
range from 0.5 to 0.8; (iii) .phi..sub.10 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 1 to 12 .mu.m.
Aspect 1c: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.7-1.4; (ii) .phi..sub.a is in the
range from 0.5 to 0.8; (iii) .phi..sub.10 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 2 to 8 .mu.m.
Aspect 1d: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.7-1.4; (ii) .phi..sub.a is in the
range from 0.5 to 0.8; (iii) (P is at least 0.8; (iv) the D.sub.50
particle size of the porous carbon framework is in the range from 2
to 8 .mu.m.
Aspect 1e: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.8-1.2; (ii) .phi..sub.a is in the
range from 0.6 to 0.8; (iii) .phi..sub.10 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 1 to 18 .mu.m.
Aspect 1f: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.8-1.2; (ii) .phi..sub.a is in the
range from 0.6 to 0.8; (iii) .phi..sub.10 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 1 to 12 .mu.m.
Aspect 1g: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.8-1.2; (ii) .phi..sub.a is in the
range from 0.6 to 0.8; (iii) .phi..sub.10 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 2 to 8 .mu.m.
Aspect 1h: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.8-1.2; (ii) .phi..sub.a is in the
range from 0.6 to 0.8; (iii) .phi..sub.5 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 2 to 8 .mu.m.
Aspect 1i: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.7-1.4; (ii) .phi..sub.a is in the
range from 0.5 to 0.8; (iii) .phi..sub.10 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 1 to 18 .mu.m; (v) the weight ratio of silicon to the
porous carbon framework is at least the value given by
[.phi..sub.b+0.8].times.P.sub.1, and preferably no more than the
value given by [.phi..sub.b+1.6].times.P.sub.1.
Aspect 1j: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.7-1.4; (ii) .phi..sub.a is in the
range from 0.5 to 0.8; (iii) .phi..sub.10 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 1 to 12 .mu.m; (v) the weight ratio of silicon to the
porous carbon framework is at least the value given by
[.phi..sub.b+0.8].times.P.sub.1, and preferably no more than the
value given by [.phi..sub.b+1.6].times.P.sub.1.
Aspect 1k: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.7-1.4; (ii) .phi..sub.a is in the
range from 0.5 to 0.8; (iii) .phi..sub.10 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 2 to 8 .mu.m (v) the weight ratio of silicon to the
porous carbon framework is at least the value given by
[.phi..sub.b+0.8].times.P.sub.1, and preferably no more than the
value given by [.phi..sub.b+1.6].times.P.sub.1.
Aspect 1l: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.7-1.4; (ii) .phi..sub.a is in the
range from 0.5 to 0.8; (iii) .phi..sub.5 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 2 to 8 .mu.m; (v) the weight ratio of silicon to the
porous carbon framework is at least the value given by
[.phi..sub.b+0.8].times.P.sub.1, and preferably no more than the
value given by [.phi..sub.b+1.6].times.P.sub.1.
Aspect 1m: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.8-1.2; (ii) .phi..sub.a is in the
range from 0.6 to 0.8; (iii) .phi..sub.10 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 1 to 18 .mu.m; (v) the weight ratio of silicon to the
porous carbon framework is at least the value given by
[.phi..sub.b+0.9].times.P.sub.1, and preferably no more than the
value given by [.phi..sub.b+1.5].times.P.sub.1.
Aspect 1n: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.8-1.2; (ii) .phi..sub.a is in the
range from 0.6 to 0.8; (iii) .phi..sub.10 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 1 to 12 .mu.m; (v) the weight ratio of silicon to the
porous carbon framework is at least the value given by
[.phi..sub.b+0.9].times.P.sub.1, and preferably no more than the
value given by [.phi..sub.b+1.5].times.P.sub.1.
Aspect 1o: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.8-1.2; (ii) .phi..sub.a is in the
range from 0.6 to 0.8; (iii) .phi..sub.10 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 2 to 8 .mu.m; (v) the weight ratio of silicon to the
porous carbon framework is at least the value given by
[.phi..sub.b+0.9].times.P.sub.1, and preferably no more than the
value given by [.phi..sub.b+1.5].times.P.sub.1.
Aspect 1p: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.8-1.2; (ii) .phi..sub.a is in the
range from 0.6 to 0.8; (iii) (P.sub.5 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 2 to 8 .mu.m; (v) the weight ratio of silicon to the
porous carbon framework is at least the value given by
[.phi..sub.b+0.9].times.P.sub.1, and preferably no more than the
value given by [.phi..sub.b+1.5].times.P.sub.1.
Aspect 1q: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.8-1.2; (ii) .phi..sub.a is in the
range from 0.6 to 0.8; (iii) .phi..sub.10 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 1 to 18 .mu.m; (v) the weight ratio of silicon to the
porous carbon framework is at least the value given by
[.phi..sub.b+1].times.P.sub.1, and preferably no more than the
value given by [.phi..sub.b+1.5].times.P.sub.1.
Aspect 1r: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.8-1.2; (ii) .phi..sub.a is in the
range from 0.6 to 0.8; (iii) .phi..sub.10 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 1 to 12 .mu.m; (v) the weight ratio of silicon to the
porous carbon framework is at least the value given by
[.phi..sub.b+1].times.P.sub.1, and preferably no more than the
value given by [.phi..sub.b+1.5].times.P.sub.1.
Aspect 1s: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.8-1.2; (ii) .phi..sub.a is in the
range from 0.6 to 0.8; (iii) .phi..sub.10 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 2 to 8 .mu.m; (v) the weight ratio of silicon to the
porous carbon framework is at least the value given by
[.phi..sub.b+1].times.P.sub.1, and preferably no more than the
value given by [.phi..sub.b+1.5].times.P.sub.1.
Aspect 1t: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.8-1.2; (ii) .phi..sub.a is in the
range from 0.6 to 0.8; (iii) .phi..sub.5 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 2 to 8 .mu.m; (v) the weight ratio of silicon to the
porous carbon framework is at least the value given by
[.phi..sub.b+1].times.P.sub.1, and preferably no more than the
value given by [.phi..sub.b+1.5].times.P.sub.1.
Aspect 1u: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.7-1.4; (ii) .phi..sub.a is in the
range from 0.5 to 0.8; (iii) .phi..sub.10 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 1 to 18 .mu.m; (v) the weight ratio of silicon to the
porous carbon framework is in the range from [1.2.times.P.sub.1 to
1.8.times.P.sub.1]:1.
Aspect 1v: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.7-1.4; (ii) .phi..sub.a is in the
range from 0.5 to 0.8; (iii) .phi..sub.10 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 1 to 12 .mu.m; (v) the weight ratio of silicon to the
porous carbon framework is in the range from [1.2.times.P.sub.1 to
1.8.times.P.sub.1]:1.
Aspect 1w: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.7-1.4; (ii) .phi..sub.a is in the
range from 0.5 to 0.8; (iii) .phi..sub.10 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 2 to 8 .mu.m (v) the weight ratio of silicon to the
porous carbon framework is in the range from [1.2.times.P.sub.1 to
1.8.times.P.sub.1]:1.
Aspect 1x: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.7-1.4; (ii) .phi..sub.a is in the
range from 0.5 to 0.8; (iii) .phi..sub.5 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 2 to 8 .mu.m; (v) the weight ratio of silicon to the
porous carbon framework is in the range from [1.2.times.P.sub.1 to
1.8.times.P.sub.1]:1.
Aspect 1y: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.8-1.2; (ii) .phi..sub.a is in the
range from 0.6 to 0.8; (iii) .phi..sub.10 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 1 to 18 .mu.m; (v) the weight ratio of silicon to the
porous carbon framework is in the range from [1.2.times.P.sub.1 to
1.6.times.P.sub.1]:1.
Aspect 1z: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.8-1.2; (ii) .phi..sub.a is in the
range from 0.6 to 0.8; (iii) .phi..sub.10 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 1 to 12 .mu.m; (v) the weight ratio of silicon to the
porous carbon framework is in the range from [1.2.times.P.sub.1 to
1.6.times.P.sub.1]:1.
Aspect 1aa: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.8-1.2; (ii) .phi..sub.a is in the
range from 0.6 to 0.8; (iii) .phi..sub.10 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 2 to 8 .mu.m; (v) the weight ratio of silicon to the
porous carbon framework is in the range from [1.2.times.P.sub.1 to
1.6.times.P.sub.1]:1.
Aspect 1bb: A particulate material as described above, wherein: (i)
P.sub.1 is in the range from 0.8-1.2; (ii) .phi..sub.a is in the
range from 0.6 to 0.8; (iii) .phi..sub.5 is at least 0.8; (iv) the
D.sub.50 particle size of the porous carbon framework is in the
range from 2 to 8 .mu.m; (v) the weight ratio of silicon to the
porous carbon framework is in the range from [1.2.times.P.sub.1 to
1.6.times.P.sub.1]:1.
In accordance with the present invention, it is to be understood
that the preferred/optional features disclosed herein in relation
to the first aspect of the invention that fall within the scope of
the above-described aspects 1a to 1bb are also to be taken as
preferred/optional features of the aspects 1a to 1bb. Likewise, any
features of the dependent claims that fall within the scope of the
above-described aspects 1a to 1 bb are also to be interpreted as
though those claims also depended from aspects 1a to 1 bb.
In a second aspect of the invention, there is provided a
composition comprising a particulate material according to the
first aspect of the invention and at least one other component. In
particular, the particulate material of the first aspect of the
invention may be used as a component of an electrode composition.
The particulate material used to prepare the composition of the
second aspect of the invention may have any of the features
described as preferred or optional with regard to the first aspect
of the invention, and may be a particulate material according to
any of aspects 1a to 1 bb.
Thus, there is provided a composition comprising a particulate
material according to the first aspect of the invention and at
least one other component selected from: (i) a binder; (ii) a
conductive additive; and (iii) an additional particulate
electroactive material.
The particulate material used to prepare the composition of the
second aspect of the invention may have any of the features
described as preferred or optional with regard to the first aspect
of the invention.
The composition is preferably a hybrid electrode composition which
comprises a particulate material according to the first aspect of
the invention and at least one additional particulate electroactive
material.
The at least one additional particulate electroactive material
preferably has a specific capacity on lithiation in the range from
100 to 600 mAh/g, or from 200 to 500 mAh/g, or from 200 to 500
mAh/g. Examples of additional particulate electroactive materials
include graphite, hard carbon, silicon, tin, germanium, gallium,
aluminium and lead. The at least one additional particulate
electroactive material is preferably selected from graphite and
hard carbon, and most preferably the at least one additional
particulate electroactive material is graphite.
The at least one additional particulate electroactive material
preferably has a D.sub.50 particle diameter in the range from 10 to
50 .mu.m, preferably from 10 to 40 .mu.m, more preferably from 10
to 30 .mu.m and most preferably from 10 to 25 .mu.m, for example
from 15 to 25 .mu.m.
The D.sub.10 particle diameter of the at least one additional
particulate electroactive material is preferably at least 5 .mu.m,
more preferably at least 6 .mu.m, more preferably at least 7 .mu.m,
more preferably at least 8 .mu.m, more preferably at least 9 .mu.m,
and still more preferably at least 10 .mu.m.
The D.sub.90 particle diameter of the at least one additional
particulate electroactive material is preferably no more than 100
.mu.m, more preferably no more than 80 .mu.m, more preferably no
more than 60 .mu.m, more preferably no more than 50 .mu.m, and most
preferably no more than 40 .mu.m.
The at least one additional particulate electroactive material is
preferably selected from graphite and hard carbon particles having
a D.sub.50 particle diameter in the range from 10 to 50 .mu.m.
Still more preferably, the at least one additional particulate
electroactive material is selected from graphite particles, wherein
the graphite particles have a D.sub.50 particle diameter in the
range from 10 to 50 .mu.m.
The at least one additional particulate electroactive material is
preferably in the form of spheroidal particles having an average
sphericity of at least 0.70, preferably at least 0.85, more
preferably at least 0.90, more preferably at least 0.92, more
preferably at least 0.93, more preferably at least 0.94, and most
preferably at least 0.95.
The at least one additional particulate electroactive material
preferably has an average aspect ratio of less than 3:1, preferably
no more than 2.5:1, more preferably no more than 2:1, more
preferably no more than 1.8:1, more preferably no more than 1.6:1,
more preferably no more than 1.4:1 and most preferably no more than
1.2:1.
The particulate material of the invention may constitute from 0.5
to 80 wt % of the total dry weight of the electroactive materials
in the composition. For instance, the particulate material of the
invention may constitute from 2 to 70 wt %, or from 4 to 60 wt %,
or from 5 to 50 wt % of the total dry weight of the electroactive
materials in the composition.
In the case that the composition is a hybrid electrode composition
comprising at least one additional particulate electroactive
material as described above, the electrode composition preferably
comprises from 1 to 20 wt %, or from 2 to 15 wt %, or from 2 to 10
wt %, or from 2 to 5 wt % of the particulate material of the
invention, based on the total dry weight of the composition.
Furthermore, in the case that the composition is a hybrid electrode
composition, the electrode composition preferably comprises from 10
to 98 wt %, or from 15 to 97 wt %, or from 20 to 97 wt %, or from
25 to 97 wt % of the at least one additional particulate
electroactive material, based on the total dry weight of the
composition.
The ratio of the at least one additional particulate electroactive
material to the particulate material of the invention is suitably
in the range from 50:50 to 99:1 by weight, more preferably from
60:40 to 98:2 by weight, more preferably 70:30 to 97:3 by weight,
more preferably 80:20 to 96:4 by weight, and most preferably 85:15
to 95:5 by weight.
The at least one additional particulate electroactive material and
the particulate material of the invention together preferably
constitute at least 50 wt %, more preferably at least 60% by weight
of, more preferably at least 70 wt %, and most preferably at least
80 wt %, for example at least 85 wt %, at least 90 wt %, or at
least 95 wt % of the total weight of the composition.
The composition may optionally comprise a binder. A binder
functions to adhere the composition to a current collector and to
maintain the integrity of the composition. Examples of binders
which may be used in accordance with the present invention include
polyvinylidene fluoride (PVDF), polyacrylic acid (PAA) and alkali
metal salts thereof, modified polyacrylic acid (mPAA) and alkali
metal salts thereof, carboxymethylcellulose (CMC), modified
carboxymethylcellulose (mCMC), sodium carboxymethylcellulose
(Na-CMC), polyvinylalcohol (PVA), alginates and alkali metal salts
thereof, styrene-butadiene rubber (SBR) and polyimide. The
composition may comprise a mixture of binders. Preferably, the
binder comprises polymers selected from polyacrylic acid (PAA) and
alkali metal salts thereof, and modified polyacrylic acid (mPAA)
and alkali metal salts thereof, SBR and CMC.
The binder may suitably be present in an amount of from 0.5 to 20
wt %, preferably 1 to 15 wt % and most preferably 2 to 10 wt %,
based on the total dry weight of the composition.
The binder may optionally be present in combination with one or
more additives that modify the properties of the binder, such as
cross-linking accelerators, coupling agents and/or adhesive
accelerators.
The composition may optionally comprise one or more conductive
additives. Preferred conductive additives are non-electroactive
materials which are included so as to improve electrical
conductivity between the electroactive components of the electrode
composition and between the electroactive components of the
electrode composition and a current collector. The conductive
additives may suitably be selected from carbon black, carbon
fibers, carbon nanotubes, graphene, acetylene black, ketjen black,
metal fibers, metal powders and conductive metal oxides. Preferred
conductive additives include carbon black and carbon nanotubes.
The one or more conductive additives may suitably be present in a
total amount of from 0.5 to 20 wt %, preferably 1 to 15 wt % and
most preferably 2 to 10 wt %, based on the total dry weight of the
electrode composition.
In a third aspect, the invention provides an electrode comprising a
particulate material as defined with reference to the first aspect
of the invention in electrical contact with a current collector.
The particulate material used to prepare the electrode of the third
aspect of the invention may have any of the features described as
preferred or optional with regard to the first aspect of the
invention, and may be a particulate material according to any of
aspects 1a to 1bb.
As used herein, the term current collector refers to any conductive
substrate which is capable of carrying a current to and from the
electroactive particles in the electrode composition. Examples of
materials that can be used as the current collector include copper,
aluminium, stainless steel, nickel, titanium and sintered carbon.
Copper is a preferred material. The current collector is typically
in the form of a foil or mesh having a thickness of between 3 to
500 .mu.m. The particulate materials of the invention may be
applied to one or both surfaces of the current collector to a
thickness which is preferably in the range from 10 .mu.m to 1 mm,
for example from 20 to 500 .mu.m, or from 50 to 200 .mu.m.
Preferably, the electrode comprises an electrode composition as
defined with reference to the second aspect of the invention in
electrical contact with a current collector. The electrode
composition may have any of the features described as preferred or
optional with regard to the second aspect of the invention.
The electrode of the third aspect of the invention may suitably be
fabricated by combining the particulate material of the invention
(optionally in the form of the electrode composition of the
invention) with a solvent and optionally one or more viscosity
modifying additives to form a slurry. The slurry is then cast onto
the surface of a current collector and the solvent is removed,
thereby forming an electrode layer on the surface of the current
collector. Further steps, such as heat treatment to cure any
binders and/or calendaring of the electrode layer may be carried
out as appropriate. The electrode layer suitably has a thickness in
the range from 20 .mu.m to 2 mm, preferably 20 .mu.m to 1 mm,
preferably 20 .mu.m to 500 .mu.m, preferably 20 .mu.m to 200 .mu.m,
preferably 20 .mu.m to 100 .mu.m, preferably 20 .mu.m to 50
.mu.m.
Alternatively, the slurry may be formed into a freestanding film or
mat comprising the particulate material of the invention, for
instance by casting the slurry onto a suitable casting template,
removing the solvent and then removing the casting template. The
resulting film or mat is in the form of a cohesive, freestanding
mass which may then be bonded to a current collector by known
methods.
The electrode of the third aspect of the invention may be used as
the anode of a metal-ion battery. Thus, in a fourth aspect, the
present invention provides a rechargeable metal-ion battery
comprising an anode, the anode comprising an electrode as described
above, a cathode comprising a cathode active material capable of
releasing and reabsorbing metal ions; and an electrolyte between
the anode and the cathode. The particulate material used to prepare
the battery of the fourth aspect of the invention may have any of
the features described as preferred or optional with regard to the
first aspect of the invention, and may be a particulate material
according to any of aspects 1a to 1 bb.
The metal ions are preferably lithium ions. More preferably the
rechargeable metal-ion battery of the invention is a lithium-ion
battery, and the cathode active material is capable of releasing
and lithium ions.
The cathode active material is preferably a metal oxide-based
composite. Examples of suitable cathode active materials include
LiCoO.sub.2, LiCo.sub.0.99Al.sub.0.01O.sub.2, LiNiO.sub.2,
LiMnO.sub.2, LiCo.sub.0.5Ni.sub.0.5O.sub.2,
LiCo.sub.0.7Ni.sub.0.3O.sub.2, LiCo.sub.0.8Ni.sub.0.2O.sub.2,
LiCo.sub.0.82Ni.sub.0.18O.sub.2,
LiCo.sub.0.8Ni.sub.0.15Al.sub.0.05O.sub.2,
LiNi.sub.0.4Co.sub.0.3Mn.sub.0.3O.sub.2 and
LiNi.sub.0.33Co.sub.0.33Mn.sub.0.34O.sub.2. The cathode current
collector is generally of a thickness of between 3 to 500 .mu.m.
Examples of materials that can be used as the cathode current
collector include aluminium, stainless steel, nickel, titanium and
sintered carbon.
The electrolyte is suitably a non-aqueous electrolyte containing a
metal salt, e.g. a lithium salt, and may include, without
limitation, non-aqueous electrolytic solutions, solid electrolytes
and inorganic solid electrolytes. Examples of non-aqueous
electrolyte solutions that can be used include non-protic organic
solvents such as propylene carbonate, ethylene carbonate, butylene
carbonates, dimethyl carbonate, diethyl carbonate, gamma
butyrolactone, 1,2-dimethoxyethane, 2-methyltetrahydrofuran,
dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide,
acetonitrile, nitromethane, methylformate, methyl acetate,
phosphoric acid triesters, trimethoxymethane, sulfolane, methyl
sulfolane and 1,3-dimethyl-2-imidazolidinone.
Examples of organic solid electrolytes include polyethylene
derivatives polyethyleneoxide derivatives, polypropylene oxide
derivatives, phosphoric acid ester polymers, polyester sulfide,
polyvinylalcohols, polyvinylidine fluoride and polymers containing
ionic dissociation groups.
Examples of inorganic solid electrolytes include nitrides, halides
and sulfides of lithium salts such as Li.sub.5NI.sub.2, Li.sub.3N,
LiI, LiSiO.sub.4, Li.sub.2SiS.sub.3, Li.sub.4SiO.sub.4, LiOH and
Li.sub.3PO.sub.4.
The lithium salt is suitably soluble in the chosen solvent or
mixture of solvents. Examples of suitable lithium salts include
LiCl, LiBr, LiI, LiClO.sub.4, LiBF.sub.4, LiBC.sub.4O.sub.8,
LiPF.sub.6, LiCF.sub.3SO.sub.3, LiAsF.sub.6, LiSbF.sub.6,
LiAlCl.sub.4, CH.sub.3SO.sub.3Li and CF.sub.3SO.sub.3Li.
Where the electrolyte is a non-aqueous organic solution, the
metal-ion battery is preferably provided with a separator
interposed between the anode and the cathode. The separator is
typically formed of an insulating material having high ion
permeability and high mechanical strength. The separator typically
has a pore diameter of between 0.01 and 100 .mu.m and a thickness
of between 5 and 300 .mu.m. Examples of suitable electrode
separators include a micro-porous polyethylene film.
The separator may be replaced by a polymer electrolyte material and
in such cases the polymer electrolyte material is present within
both the composite anode layer and the composite cathode layer. The
polymer electrolyte material can be a solid polymer electrolyte or
a gel-type polymer electrolyte.
In a fifth aspect, the invention provides the use of a particulate
material as defined with reference to the first aspect of the
invention as an anode active material. Preferably, the particulate
material is in the form of an electrode composition as defined with
reference to the second aspect of the invention, and most
preferably the electrode composition comprises one or more
additional particulate electroactive materials as defined above.
The particulate material used according to the fifth aspect of the
invention may have any of the features described as preferred or
optional with regard to the first aspect of the invention, and may
be a particulate material according to any of aspects 1a to 1
bb.
* * * * *
References